The present invention relates to the field of antennas, and, more particularly, to a phase shifter for a phased array antenna.
Phased array antennas are well known, and are commonly used in satellite, electronic warfare, radar and communication systems. A phased array antenna includes a plurality of antenna elements and respective phase shifters that can be adjusted for producing a steerable antenna beam in a desired direction.
A scanning phased array antenna steers or scans the direction of the RF signal being transmitted without physically moving the antenna. Likewise, the scanning phased array antenna can be steered or scanned without physically moving the antenna so that the main beam of the phased array antenna is in the desired direction for receiving an RF signal. This enables directed communications in which the RF signal is electronically focused in the desired direction.
One type of phase shifter includes switching diodes and transistors that change the path length, and thus the phase shift through the phase shifter via bias current changes. Another type phase shifter includes a phase shifting material that produces a phase shift via a DC static voltage applied across the material. A variable voltage applied to the phase shifting material induces a change in its dielectric constant. As a result, an RF signal being conducted through the transmission line phase shifter exhibits a variable phase delay. In other words, the electrical length of the transmission line can be changed by varying the applied voltage.
A conventional phase shifter 10 will now be discussed with reference to FIG. 1. The prior art phase shifter 10 includes an RF signal input path 12 and an RF signal output path 14. A phase shifting material 16 is between the RF signal input and output paths 12, 14. A bias network 18 is connected to the phase shifting material 16 for applying a voltage thereto for controlling the dielectric constant.
A respective impedance matching network 20 is required to match the impedance of the phase shifting material 16, and the RF signal input and output paths 12, 14. The transmission line when loaded by the phase shifting material 16 typically has a low impedance in a range of about 1 to 10 ohms, whereas the impedance of the RF signal input and output paths 12, 14 is about 50 ohms. Consequently, the two impedance matching networks 20 are required.
However, a problem arises where space and power are at a premium, particularly in airborne platforms. A typical phased array antenna requires several thousand antenna elements, each with its own phase shifter. The impedance matching networks 20 required for each phase shifter 10 increases the length of the phase shifter by a factor of 4 as compared to the phase shifting material 16 alone. For example, the phase shifting material 16 has a dielectric constant of about 400 and is typically about 0.4 inches in length for an RF signal having an operating frequency of 10 GHz, but with the addition of the impedance matching networks 20, the overall length of the phase shifter 10 may be increased to about 2.4 inches. Moreover, it is readily understood by those skilled in the art that the length of the phase shifter may be calculated by recognizing that 0.4 inches in length will change the insertion phase by 10% of its length.
In addition to the impedance matching networks 20 adding to the physical size and weight of each transmission line phase shifter 10, attenuation losses of the RF signal being conducted through the transmission line phase shifter also increase. Consequently, a larger drive voltage is required to overcome the losses introduced by the impedance matching networks 20. This in turn adds to the overall cost of each transmission line phase shifter 10.
Unfortunately, phased array antennas are limited in their application primarily by cost. Even using the latest monolithic microwave integrated circuit (MMIC) technology, an individual phase shifter may have a unit cost in excess of $500. With a typical phased array antenna requiring several thousand antenna elements, each with its own phase shifter, the price of the phased array antenna quickly becomes very expensive.
In view of the foregoing background, it is therefore an object of the present invention to provide a phase shifter that is smaller in size as compared to a conventional phase shifter.
Another object of the present invention is to provide a phase shifter with reduced RF signal attenuation losses as compared to a conventional phase shifter.
A further object of the present invention is to provide a phased array antenna at a significantly lower cost than a conventional phased array antenna.
Yet another object of the present invention is to provide a method for making a phase shifter that overcomes size and attenuation losses introduced with a conventional phase shifter.
These and other objects, advantages and features in accordance with the present invention are provided by a transmission line phase shifter comprising a substrate, and first and second conductive portions adjacent the substrate with a gap therebetween. The first and second conductive portions define a signal path. A body comprising a phase shifting material is preferably in the gap and has a controllable dielectric constant for causing a phase shift of a signal through the signal path.
The body preferably has an enlarged width medial portion tapering downwards in width towards respective end portions for impedance matching with the first and second conductive portions. The width of the tapered end portions of the body are preferably selected so that a separate impedance matching network is not required for impedance matching with the first and second conductive portions.
The body in accordance with the present invention advantageously combines the functions of phase shifting the signal being conducted therethrough and impedance matching with the first and second conductive portions. The first and second conductive portions each preferably has an impedance of about 50 ohms. The enlarged width medial portion of the phase shifting material body preferably has an impedance in a range of about 1 to 10 ohms.
In other words, the width of the tapered end portions of the phase shifting material body are preferably selected so that a separate impedance matching network is not required for impedance matching with the first and second conductive portions. The opposing ends of the first and second conductive portions adjacent the gap also preferably have a reduced width that corresponds to a width of the end portions of the body. Because an impedance matching network is not required, the length of the phase shifter may be significantly reduced by at least a factor of 4. This allows construction of a lower cost, much smaller and lower loss phase shifter.
In one embodiment, the body preferably comprises a substrate with a layer of the phase shifting material thereon. In another embodiment, the body comprises a bulk phase shifting material body.
The phase shifting material preferably comprises a ferroelectric material, such as barium strontium titanate, or a ferromagnetic material. The body may have an overall thickness equal to or greater than about 0.002 inches. Because the body has a thickness that is relatively easy to handle, the body may be simply bonded to the substrate exposed by the gap between the first and second conductive portions.
Consequently, in forming a phased array antenna, the bodies are preferably loaded into production surface mount or similar machines. The present invention is thus very adaptable to mass production using techniques as readily understood by one skilled in the art.
Each phase shifter preferably further comprises at least one third conductive portion adjacent the substrate for defining a ground structure. In one embodiment, the at least one third conductive portion preferably comprises a pair of laterally spaced apart third conductive portions along opposing sides of the signal path. This defines a coplanar waveguide structure. Each of the pair of laterally spaced apart third conductive portions may also have a recess adjacent and corresponding to the enlarged width medial portion of the body. In another embodiment, the signal path vertically extends from the third conductive portion for defining a microstrip structure.
Another aspect of the invention relates to a method for making a phase shifter. The method preferably comprises forming first and second conductive portions adjacent a substrate with a gap therebetween. The first and second conductive portions define a signal path.
The method further preferably includes inserting a body in the gap. The body preferably comprises a phase shifting material having a controllable dielectric constant for causing a phase shift of a signal through the signal path. The phase shifting material body preferably has an enlarged width medial portion tapering downwards in width towards respective end portions for impedance matching with the first and second conductive portions.
In one embodiment, the body may have a diamond shape. Inserting body may be performed using a surface mount machine. Each body may also have a thickness equal to or greater than about 0.002 inches.